Method of electrochemical machining

ABSTRACT

A METHOD OF ELECTROCHEMICAL MACHINING WORKPIECES TO FORM SURFACES OF REVOLUTION HAVING PRECISE SHAPES AND DIMENSIONS INCLUDES THE STEPS OF ROTATING THE WORKPIECE AND POSITIONING AN ELECTROCHEMICAL MACHINING TOOL ADJACENT THE WORKPIECE WITH THE GAP BETWEEN THE TOOL AND THE WORKPIECE BEING ADJUSTED INITIALLY TO A FIRST DISTANCE. A HIGH VELOCITY FLOW OF ELECTROLYTE IS FORCED UNDER PRESSURE INTO THE GAP BETWEEN THE TOOL AND THE WORKPIECE TO PROVIDE A PATH FOR THE ELECTRICAL CURRENT, TO REMOVE THE HEAT GENERATED DURING THE MACHINING OPERATION, AND TO REMOVE THE REACTION PRODUCTS OF THE MACHINING OPERATION. THE CURRENT DENSITY BETWEEN THE WORKPIECE AND THE TOOL IS MAINTAINED AT THE HIGHEST POSSI-   BLE LEVEL FOR SUBSTANTIALLY THE ENTIRE MACHINING OPERATION, IN THE ORDER OF 6000 AMPERES PER SQUARE INCH, TO OBTAIN THE HIGHEST QUALITY SURFACE FINISH AND THE BEST MACHINING RATES. AFTER THE WORKPIECE HAS BEEN MACHINED TO ITS FINISH DIMENSION, THE CURRENT MAY BE REDUCED MOMENTARILY, FOR AT LEAST ONE REVOLUTION OF THE WORKPIECE TO A LOWER PREDETERMINED LEVEL, TYPICALLY BETWEEN 1500 AND 3000 AMPERES PER SQUARE INCH, TO PRODUCE A BRIGHT, PLEASING APPEARANCE TO BE MACHINED SURFACE. THE CURRENT IS THEN TERMINATED ABRUPTLY IN ORDER TO MAINTAIN THIS BRIGHT SURFACE APPEARANCE COMPLETELY AROUND THE BEARING SURFACE.

1, 3 W. A. HAGGERTY 3,730,861

METHOD OF ELECTROCHEMICAL MACHINING Filed April 8, 1968 v 5 Sheets-Sheet1 FIG-1 25 f I? Ta 7' W 12 I 73 o O O 0 Q Q o Y 65 "WHW' MW JmHnw'WILLIAM ANDREW HAGGERTY I M, M?

. 1973 w. A. HAGGERTY 3,730,861

METHOD OF. ELECTROCHEMICAL MACHINING Filed April 8, 1968 3 Sheets-Sheet3 Filed April 8, 1968 3 Sheets-Sheet 3 52 IIIIIIIHHWHIHW.

FIG 11 [VOLTAGE CURRENT Patented May 1, 1973 3,730,861 METHOD OFELECTROCHEMICAL MACHINING William Andrew Haggerty, Cincinnati, Ohio,assignor to The Cincinnati Milling Machine Co., Cincinnati, Ohio' FiledApr. 8, 1968, Ser. No. 719,450 Int. Cl. C23b 3/02, 3/12; B23k 1/00 U.S.Cl. 204129.75 9 Claims ABSTRACT OF THE DISCLOSURE distance. A highvelocity flow of electrolyte is forced under pressure into the gapbetween the tool and the workpiece to provide a path for the electricalcurrent, to remove the heat generated during the machining operation,and to remove the reaction products of the machining operation. Thecurrent density between the workpiece and the tool is maintained at thehighest possible level for substantially the entire machining operation,in the order of 6000 amperes per square inch, to obtain the highestquality surface finish and the best machining rates. After the workpiecehas been machined to its finish dimension, the current may be reducedmomentarily, for at least one revolution of the workpiece to a lowerpredetermined level, typically between 1500 and 3000 amperes per squareinch, to produce a bright, pleasing appearance to be machined surface.The current is then terminated abruptly in order to maintain this brightsurface appearance completely around the bearing surface.

RELATED APPLICATIONS Reference is hereby made to copending United Statesapplication Ser. No. 719,451, new U.S. Pat. 3,591,493, entitled Methodand Apparatus for Electrochemically Machining Rotating Parts, and Ser.No. 719,452 entitled Method and Apparatus for ElectrochemicallyMachining Rotating Parts, both applications filed on even date herewith.

BACKGROUND OF THE INVENTION In the preparation of the bearing races,usually the bearing is first formed by turning on a screw machine andthen heat treated to carburize the outermost surface layer. The outsidefaces of the bearing are then ground parallel to each other to definethe total length of the bearing. Finally, the bearing surface is roughground to approximately the desired outside diameter, finish ground, andthen honed to obtain proper surface finish and diameter. Each of thesethree last mentioned steps requires separate machining operations.

It has been found that these several grinding operations can beeliminated or reduced by using the electrochemical machining process,and in this example the rough or finish grinding as well as the honingoperation can be replaced and more accurately and more quicklyaccomplished by using electrochemical machining. Furthermore, morecomplicated surface configurations can be obtained by electrochemicalmachining, such as crowning the bearing surface to increase the loadcarrying capacity of a bearing, with each part machined having exactlythe same configuration as every other part since the tool which does themachining is not worn or in any way modified during successive machiningoperations.

It has also been found that the highest machining tainable only atrelatively high current densities, in the order of 6000 amperes persquare inch. However, at such high current densities, the resultantsurface has a hazy, straw-colored appearance. At intermediate currentdensities, typically in the order of 1500 to 3000 amperes per squareinch, the surface finish is bright, having no apparent oxide coatingthereon, however, this current density does not produce the fastestcutting rate nor the smoothest finishes possible from theelectrochemical machining process. At lower current densities, an oxideforms on the surface of the part being machined which gives the part ablack, dirty appearance.

SUMMARY OF THE INVENTION This invention relates to a method forelectrochemically machining a workpiece to form a surface of revolutionto precise dimensions and to smooth surface finishes which have a brightappearance, and which have no apparent oxide coating thereon.

More particularly, the method of this invention includes the steps ofrotating a workpiece about a center of rotation, placing anelectrochemical machining tool adjacent the workpiece and initiallyadjusting the gap between the tool and the workpiece, supplying a highvelocity flow of electrolyte under pressure into the gap to provide apath for the electrical current flow and to remove the heat and thereaction products of the electrochemical machining operation,maintaining the magnitude of the current flow at the highest possiblecurrent level, typically in the order of 6000 amperes per square inch,to provide high machining rates and a smooth surface finish, maintainingthat current density until the workpiece has been machined to itsdesired dimensions, and then momentarily lowering the current density toa second predetermined level, typically between 1500 and 3000 amperesper square inch, for at least one revolution of the workpiece to providea bright surface finish. Since current densities lower thanapproximately 1500 amperes per square inch will leave a black oxidecoating on the workpiece, it is essential that the current density belowered immediately to a zero value at the termination of the machiningoperation, otherwise a black line will appear on the rotating part as aresult of the lower current density during the time the power supply isbeing disconnected.

Since the highest possible current densities provide the fastestmachining rates and the smoothest surface finishes, these currentdensities are used in the method of this invention with the maximumcurrent density being determined primarily by the current carryingcapacity of the tool.

rates and the smoothest possible surface finish are ob- Among theconsiderations given in limiting the maximum current flow is the factthat when the workpiece is initially installed for rotation, it may beout of round thereby causing the gap distance between the workpiece andthe tool to vary considerably as the workpiece is rotated. Also, adifferential taper may exist between the tool and the workpiece causingthe gap distance to vary across the face of the tool. Since currentdensity is a function of gap distance for any given voltage, the currentthrough the tool must be limited to a value below that which causesdamage to the tool from melting, arcing, or distortion caused by theheat generated by the passage of current through the tool. By riding onsupport shoes, the initial operation efiects rounding out of theworkpiece with a minimum removal of stock.

Preferably, a gap distance of approximately 0.0015 inch is establishedbetween the tool and the workpiece at the completion of the machiningoperation. At this gap dimen sion, 6000 ampere per square inch currentdensity, the highest current density deemed safe for the particular tooldescribed in this application, can be obtained by maintaining a voltagebetween the tool and the workpiece of approximately fifteen volts. Withsome apparatus used to perform the method of this invention, the gapdistance at the beginning of the machining operation may vary to as muchas three or four times final gap distance, and in order to maintain thesame high current density, it would be necessary to raise the voltage.Since this higher voltage may result in localized currents within thetool which exceed its current carrying capacity, the voltage, and as aresult the current through the tool, is limited, especially during theinitial rounding up of the workpiece, to a value which permits rapidremoval of workpiece material while at the same time prevents damage'tothe tool.

In the preferred embodiments, the means supplying the current for theelectrochemical machining operation is capable of maintaining anypreselected voltage even though the current may vary. This is especiallyimportant during the initial rounding up of the part since the gapdimension is constantly changing and with a smaller gap dimension highercurrent flow will result in faster machining. Thus, the high spots onthe part are removed at a faster rate and eventually the part becomesrounded. In order to maintain the surface finish at a high degree ofsmoothness throughout the circumferential extent of the machinedsurface, the power supply must be substantially ripple free, that is thevariation in its voltage output must be less than one half of onepercent, peak to peak. The power supply should also have a responsecharacteristic sufiicient to hold the voltage constant over a five toone variation of current with the frequency of this variation beingdetermined by the maximum anticipated speed of rotation of theworkpiece. With the apparatus described herein, a ten cycle per secondresponse is considered adequate.

By machining electrochemically in accordance with the method of thisinvention, the-workpiece may be machined to within a dimension of0.000100 inch, an out of round tolerance of 0.000100 inch, and to asurface finish having less than five microinch variation, arithmeticaperage, with two to two and one half microinch finishes having beenobtained in experimental runs.

The method of this invention may be practiced using at least twodifferent apparatus. While other means may be employed to rotate theworkpiece, such as rotating the workpiece between centers or by amandrel, the workpiece preferably is supported by a magnetic chuck forrotation about an axis of rotation. The center of the workpiece isdisplaced from the center of rotation by a pair of shoes engaging themachined surface of the workpiece. Thus, as the workpiece rotates, itwill have a tendency to align its center with the center of rotation,but being'displaced by the shoes, the workpiece will therefore be positively urged toward the shoes and thus its center will move toward thecenter of rotation as material is removed electrochemically from themachined surface. 7

In one apparatus, described and claimed in the above mentioned UnitedStates application Ser. No. 719,452, the electrochemical machining toolis positioned between these shoes and therefore the gap between the tooland the workpiece decreases as the machining operation progresses. Asthe gap dimension becomes smaller, the voltage between the tool and theworkpiece is reduced in order to maintain the current density at asubstantially high level, but below that level which may cause damage tothe tool. Preferably the current is maintained constant, and when thevoltage required to accomplish this isreduced to a predetermined value,indicating that the gap has been reduced to a predetermined distance,the Workpiece will be at its final dimension, Therefore, the voltagerequired to maintain the current at a constant high level is a directindication of the diameter of the workpiece and may be referred to bythe machine operator or by automatic equipment to determine when theelectrochemical machining operation is to be terminated.

In the other apparatus, described and claimed in the above mentionedUnited States application Ser. No. 719,-

' 451, the electrochemical machining tool is positioned adjacent theworkpiece on a line essentially normal to the line extending between thecenter of the workpiece and the center of rotation. As the workpiece ismachined to a smaller diameter, its center moves substantially parallelto the tool, and consequently it is desired that the essentially flatfrontal machining surface of the tool be positioned so that it will benormal to a line passing from the tool through the center of theworkpiece when the machining operation is completed. The tool may befixed relative to the. workpiece and thus allow the gap to increase asmaterial is removed, or the tool may be moved toward the workpiece atthe same rate as material removal to maintain the gap distance constant.In either case, the current density is maintained at the desired highlevel by adjusting the voltage between the tool and the workpiece.

Accordingly, it is an object of this invention to provide an improvedmethod for electrochemically machining a rotating workpiece to a smoothfinish having a bright appearance including the steps of machining theworkpiece at a high current level to provide a smooth surface finishuntil the desired dimensions are obtained, momentarily lowering thecurrent level to a predetermined level to provide a bright surfacefinish free of apparent oxide coating, and then reducing the currentdensity to a zero value to terminate the machining operation.

It is another object of this invention to provide an improved method forelectrochemically machining workpieces to form a surface of revolutionby mounting each workpiece for rotation past an electrochemicalmachining tool mounted initially a predetermined distance from theworkpiece, supplying electrolyte under pressure to produce high velocityflow between the tool and the workpiece, providing a high densityelectrical current from a substantially ripple free power supply toremove workpiece material anodically as the workpiece rotates past thetool, keeping the current density at a relatively high level until theworkpiece is machined to the desired dimensions, the current densitybeing of such magnitude that surface finishes in the order of less thanfive microinch, arithmetic average, result during the machiningoperation, lowering the current density to a predetermined magnitude forat least one revolution of the workpiece immediately prior toterminating the machining operation to remove the discoloring effectsinherently resulting from the machining at hgh current densitiestherefore to provide a bright surface finish on the workpiece, andterminating the current flow suddenly after the workpiece has beenmachined at said second predetermined level.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a front elevational viewshowing the overall arrangement of the various components which compriseone embodiment of the electrochemical machining apparatus used inperforming the method of this invention;

FIG. 2 is a front elevational view showing a portion of anotherapparatus used in performing the method of this invention;

FIG. 3 is a plan view of the electrochemical machining apparatus showingthe workpiece drive assembly, the electrochemical machining tool, and aportion of the mechanism supporting the tool;

FIG. 4 is a plan view partially in cross section of the electromagneticchuck for holding the workpiece;

FIGLS is a front elevational 'view, with the workpiece partially incross section, showing means slidingly engaging the machined surface ofthe workpiece at spaced apart locations to displace the center of theworkpiece from the center of rotation of the supporting magnetic chuck,and also showing, partially in cross-section, the electrochemicalmachining tool positioned between the sliding means;

FIG. 6 is a view similar to FIG. showing the electrochemical machiningtool positioned approximately at right angles to a line bisecting thespace between the two sliding means; I

FIG. 7 is an enlarged cross sectional elevational view of theelectrochemical machining tool used in the preferred embodiment of theinvention;

FIG. 8 is an enlarged plan view of the electrochemical machining tool;

FIG. '9 is an enlarged end view of the electrochemical machining tool;

FIG. 10 is an enlarged cross sectional view of the workpiece; and

FIG. 11 is a graph showing the voltage and current flow during themachining operation with respect to time with the curve in 'FIG. 11arepresenting the voltage between the tool and the workpiece and with thecurve in FIG. 11b representing the total current flow through the tool.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings,and to FIGS. 1 and 3 particularly, the electrochemical machiningapparatus of this invention includes an electrochemical machining tool10 mounted on a supporting means 12 which adjustably supports the toolrelative to a workpiece 15. 'In the embodiment shown, in these figures,the tool may be moved laterally by handle 16 with the position of thetool being indicated by the gage 16a, vertically by turning the handle17 with the position indicated by the gage 17a, and rearwardly by anadjusting handle 19. While the form of tool support shown isparticularly useful, it is to be understood that other means ofsupporting the tool relative to the workpiece may be employed withoutdepartihg from the scope of this invention. Once the position of thetool is adjusted properly, it will remain fixed throughout the machiningoperation.

The workpiece is supported on a magnetic chuck 20 which is carried by ashaft 21, the latter being rotated by belts reeved on pulleys 22attached to the shaft of the motor 25. A slip ring assembly 27, showngenerally in FIG. 3, carries the electrical current for theelectrochemical macnining operation through the shaft 21 to theworkpiece 15.

Referring to FIG. 4, the magnetic chuck 20 receives electrical currentthrough a slip ring assembly 28 which applies this current to a pair ofcoils 30 located within the chuck 20. The current through these coilsgenerates a magnetic field which passes from one pole of the coils 30,through the cylindrical housing 31, an outer plate 32 to an outer collar33, through the workpiece 15, an inner collar 35, an inner plate 36 andthen to the other pole of the coils 30. A ring 37 of magneticallyinsulating material separates the plates 32 and 36 and an insulatingcollar 38 separates the collars 33 and 35. This arrangement permits themaximum number of magnetic lines of force to pass through the workpiece15 thus holding it securely against the forward face 39 of the chuck 20.This face 39 is maintained relatively smooth to permit free lateralmovement of the workpiece 15.

Referring now to FIG. 5, which shows one apparatus for performing themethod of this invention, the workpiece 15 is displaced from the centerof rotation 40 of the magnetic chuck 20 by means of two spaced apartshoes 41 and 42 having a workpiece engaging surface formed to the samecontour as the surface of the workpiece 15 which will undergo theelectrochemical machining operation. These shoes are constructed of amaterial, such as tungsten carbide or ceramic, having sufiicienthardness so that they will not be appreciably worn by abrasion with thesurface of the workpiece. Alternatively, these shoes may be rollersagainst which the workpiece is urged by a roller or belt type drivermeans.

Each of the shoes 41 and 42 is pivotally attached to a radiallyextending arm 44 which may slide within a tool holder 45 and which maybe positioned radially by the screw 46. The pivotal attachment betweenthe arm 44 and the shoes permits the shoes to follow the workpiecegenerally as it is machined and moves toward the tool 10 due to thenatural tendencies for the center of the workpiece to align itself withthe center of rotation. The holders 45 are adjustably mounted within aT-shaped slot 47 formed in a plate 48 (see also FIG. 3). The plate 48has a generally circular opening in its central portion through whichthe collar 33 of the magnetic chuck 20 extends. A rubber gasket 49,shown in FIG. 4, extends from the plate 48 into the opening and engagesthe outer surface of the collar 33 to prevent electrolyte from flowingbehind the plate 48 and into the magnetic chuck 20 or the rotating partsof the chuck structure.

The holders 45 are adjusted within the slot 47 at approximately a anglewith respect to each other and the shoes 41 and 42 are moved radiallyinwardly to displace the workpiece 15 along the line which generallybisects the angle between these two shoes. Initially, the center 50 ofthe workpiece 15 is positioned approximately 0.025 inch from the centerof rotation 40 of the magnetic chuck 20 for the part hereinafterdescribed.

An electrochemical machining tool 10 is positioned approximately midwaybetween the shoes 41 and 42 and is adjusted to provide a gap of a firstpredetermined dimension, typically 0.001 inch, is provided between themachining surface of the tool and the workpiece. Since the machinedsurface of the workpiece slidably engages the shoes 41 and 42, thecenter 50 of the workpiece will be urged toward the center of rotation40, or to the right as viewed in FIG. 4, as the workpiece becomessmaller in diameter through the electrochemical removal of the workpiecematerial and thus decrease the gap distance between the tool and theworkpiece.

The tool is positioned in line with the direction of movement of thecenter of the workpiece as the latter moves toward the center ofrotation during reduction of its diameter due to the electrochemicalremoval of the workpiece material. As the gap distance decreases at anypreselected maintained voltage, the current density will normally tendto increase. To prevent the current from exceeding the capacity of thetool, the voltage between the workpiece and the tool is correspondinglydecreased either manually by the machine operator or automaticallythrough the use of constant current electronic circuits.

Another embodiment, also useful in performing the method of thisinvention, is shown in FIGS. 2 and 6. The workpiece 15 is displacedupwardly from the center of rotation 40 approximately 0.025 inch by theshoes 41 and 42. The tool 10 is positioned at approximately right anglesto the path of movement of the workpiece as the material iselectrochemically removed alnd the center of the workpiece 15 movesdownwardly toward the center of rotation 40. The tool has its frontal ormachining surface parallel to the axis of rotation and also essentiallyparallel to the direction of movement of the workpiece toward the centerof rotation.

In the embodiment shown in FIG. 6, the tool may remain stationarythroughout the machining operation, the gap between the tool and theworkpiece will increase as material is removed, and unless the voltageis increased by a corresponding amount, the current density will tend todecrease. Since it is desirable that the current density be maintainedat a substantially constant high magnitude throughout the machiningoperation to provide the smoothest possible surface finishes, thecurrent is controlled, either manually or automatically through the useof constant current circuits, by increasing the voltage. In FIG. 6, thetool also may be moved into the Workpiece at the same rate as theworkpiece material is removed, thus maintaining the gap distance at aconstant value, the current density Will inherently remain at asubstantially constant value for a given voltage level.

The electrochemical machining tool is shown in detail in FIGS. 7 through9 and includes an electrically conductive plate 51 having a frontalmachining surface 52 which is machined and lapped flat. Thiselectrically conductive plate may be made out of brass or othersimilarly easily machined metal capable of carrying high electricalcurrent levels.

An electrolyte passage 54 is provided by mounting an upper insulatingblock 55 onto the plate 51 and securing both to the tool holder by meansof screws 56. This passage communicates with an opening 57 extendingthrough the plate 51 into the tool holder 53. A passageway 58 in thetool holder carries the electrolyte from the supporting equipment intothe tool, through the opening 57 and the passage 54 into the gap 60between the tool and the workpiece.

An additional insulating block 61 is secured to the plate 51 in the areanext to the workpiece to prevent any stray electric currents frommachining the workpiece thereby limiting the machining action to thefrontal surface 52 of the tool. This block is tapered inwardly from thefrontal machining surface thus increasing the gap dimensions allowingthe electrolyte to escape from the machining area. Both the blocks 55and 61 may be formed from a rigid and non-conductive fiberglasslaminate, such as Formica type FF91, which also has low moistureabsorption characteristics.

In both the embodiments shown in FIGS. and 6, the lower edge 63 of thetool is essentially straight and is aligned parallel to the axis of theworkpiece with this edge being closer to the workpiece than any otherportion of the tool. Preferably, when using a single tool, the frontalmachining surface 52 is aligned perpendicular to the line between thecenter of the workpiece and the edge 63 of the tool when the workpiecehas been machined to its final dimension.

The lower surface of the block 55 and the upper surface of the plate 51are made relatively smooth in the area of the electrolyte passage 54 tofacilitate the smooth flow of the electrolyte into the gap 60. Also, thefrontal surface 64 of the insulating block 55 is curved or inclined asshown in FIG. 5 to provide a substantially constant gap distance andthus to urge the electrolyte to flow downwardly over the frontalmachining surface of the tool as the workpiece rotates in a clockwisedirection.

A high velocity flow of electrolyte is supplied by a pump in to the gap60 formed between the plate 51 and the workpiece at a pressure ofapproximately 350 p.s.i. as observed by the gage 65. The particularelectrolyte composition depends upon the type of material beingmachined. For iron base materials, the electrolyte solution is preparedby mixing four pounds of'sodium nitrate per gallon of water. Thiselectrolyte is maintained at substantially ambient temperature, and asit passes from the gap 60, it is collected in a tray 66 (FIG. 1) locatedbeneath the tool and returned to the recirculating equipment where theanodic products of the reaction are removed, as by a centrifugalseparator, and where the electrolyte is cooled prior to being returnedto the machining area. A shield 67 (FIG. 3) is constructed around thetool and the workpiece in order to prevent the electrolyte from beingsprayed on the machine operator and on the other components of theapparatus.

Power is supplied to the tool through its tool holder by means of cable68 and the workpiece through the slip ring assembly 27 and the shaft 21,with the workpiece being made anodic with respect to the tool. The meanssupplying the current between the tool and the workpiece is ofconventional design, but of high quality since it must supply a variabledirect current of between ieroand forty volts, and be essentially ripplefree, that is contain less than one half of one percent, peak to peak,variation in its voltage level. An essentially ripple free power supplyis necessary in order to obtain the accurate dimensioning and smoothsurface finishes necessary for machining bearings. Furthermore, thepower supply should have a response characteristic sufficient to holdthe voltage constant over a five to one variation in current, thefrequency of the variation being determined by the maximum speed ofrotation anticipated. A ten cycle per second response is consideredsufficient for the embodiment described herein.

A power supply means 70 shown in FIG. 1 includes a voltage control 71,with the voltage output being indicated by the meter 72, and the currentfiow to the tool being indicated by the ammeter 73. While manual meansare shown to adjust the voltage level, it is contemplated that automaticmeans may also be used.

The depth to which metal is removed during each revolution of theworkpiece is determined by many factors including the rate of movementof the workpiece material relative to the face of the tool, the lengthof the tool face in the direction of relative movement, the voltage andgap between the tool and the workpiece, electrolyte composition andtemperature, and the feed rate or relative radial motion between thetool and the workpiece.

In the embodiments of the invention described herein, the rate ofrotation of the workpiece and the electrolyte composition andtemperature are held constant by the supporting equipment. The currentdensity may be maintained at a substantially constant level bycontrolling the power supply voltage. Thus, if the gap becomes smaller,as is the case in the embodiment shown in FIG. 5, the voltage betweenthe tool and the workpiece is reduced during the machining operation,however, if the gap becomes larger, as may be the case in the embodimentshown in FIG. 6 if the tool is not fed toward the workpiece at the samerate that metal is removed, the voltage is increased to maintain thecurrent density at a substantially constant high pre-determined level.

The magnitude of the peak current is maintained at a first predeterminedlevel normally greater than 3000 amperes per square inch and preferablyin the order of 6000 amperes per square inch until the diameter of thebearing surface reaches the desired dimensions. The highest currentlevels are maintained within the capacity of the tool in order toprovide high rates of metal removal and a surface finish of less thanfive microinch, arithmetic average. However, a ferrous workpiecemachined at these high current levels with sodium nitrate electrolytewill have a hazy, strawlike appearance. Therefore, the current densityis lowered to a second predetermined level, typically between 1500 and3000 amperes per square inch, for at least one revolution of theworkpiece to provide a bright appearance to the surface finish. Thecurrent is then terminated quickly in order to prevent a black lineonthe surface which may occur if the electrochemical machining operationis allowed to continue at a lower current density. 4

With the apparatus shown in FIG. 5, the dimension of the workpiece maybe determined by observing the voltage level necessary to maintain aconstant current density at a first predetermined level. Therefore, whenthe voltage drops to a predetermined value, this indicates that the gap60 has been reduced to a second predetermined distance, and at thattime, the current density is reduced momentarily and then terminated.

With the apparatus shown in FIG. 6, the dimension of the workpiece isdetermined primarily by observing the readings on the dial 16a whichindicate the lateral position of the tool 10 with respect to the centerof rotation 40. In this embodiment, the tool may remain fixed relativeto the center of rotation, especially when removing only small amountsof material, and in this case the voltage required to maintain asubstantially constant current density is also an indication of theworkpiece size and may be utilized to terminate the machining operationwhen the voltage is increased to a value which can be predeterminedexperimentally.

The length of the frontal machining surface in the direction of relativemovement between the tool and the workpiece at the left end of thebearing surface 74 is made proportionately longer where the diameter ofthe workpiece is, greater and therefore where the relative rate ofmovement between the workpiece and the tool is higher. A typicalworkpiece 15, such as a bearing race, is shown in FIG. 10. Adjacent eachend of'the bearing surface 74 are two recesses 75 and 76 which serveprimarily to allow the bearing surface to be machined preciselythroughout its extent. While a conically shaped workpiece is described,it is tobe understood that any rotating workpiece may be machinedaccording to the principles outlined herein. As shown in FIG. 10, therecesses 75 and 76 adjacent the bearing surface 74 are formed byextended portions 77 and 78 at the extreme edges of the tool where thetime of exposure to the workpiece is proportionately longer.

The following table illustrates typical dimensions for the tool and theworkpiece shown in FIGS. 7 through 10.

Tool Inch A 0.800

Workpiece Inch a 3.000

The bearing surface 74 may be provided with a crown of approximately0.000050 inch to facilitate the load carrying ability of the bearing andto increase its life. Providing such a crown on the bearing surface byconventional grinding methods is possible for only a few bearings, andis therefore costly in'the production of a large number of bearingssince the grinding tool must be resurfaced frequently. Using theelectrochemical machining apparatus of this invention, the crown on thebearing surface is formed by modifying the area of the tool in thedirection of relative movement by shaping the area of the frontalmachining surface-of the tool by milling, for example, since the depthof machining is proportional to the length of the tool in the directionof relative movement.

If the tool length'is changed by 0.001 inch, then the rate of metalremoval is changed by 0.00001 inch, a factor of 100 to'1-. InFIG. 9, thesurface 79 is a curve formed on a twelve inch radius on theperpendicular bisector of the line joining the ends of the tool 51.Thus, it is apparent that accurate machining of the tool to providecomplicated surface finishes is well within the present state of theart, and the frontal machining surface of the tool is thereforemaintained flat in order to remove any variations in machining rate dueto the contour of the tool itself.- 1

The" material in the plate 51 which is cut away in order to provide thesurface configuration for machining the particular workpiece shown inthe drawings is filled with an' insulating material 80, and the topsurface of this material is machined flat with the top surface of theupwardly extending portions 77 and 78 to insure smooth electrolyte flowin the passage 55, as described above. A pluralityof holes 81 may beformed through the plate 51 in the area machined away in order to assistin bonding the insulating material 80 to the plate.

The insulating material 80 also serves to prevent stray electricalcurrent from the interior surface of the tool from degrading the surfacefinish of the workpiece. Since the distance between the workpiece andthese interior surfaces is much greater than the gap between theworkpiece and the frontal-surfaceof the tool, the current densities.from

inside the tool will be lower than from the frontal surface. If a lowercurrent density flow of current were permitted, the surface would not beas smooth as possible, and in addition the surface would have a blackappearance.

Any irregularities in the interface between the tool and the insulationor any discontinuity in the frontal surface of the tool where theinsulation joins the tool could cause a poor surface finish since theseirregularities may cause turbulence in the electrolyte flow across theface of the tube or permit stray currents to flow from an internalsurface of the tool to the workpiece. For this reason, the frontalsurface of the tool and insulation are maintained (:0- planar.

The insulating material is an epoxy type material (reaction product ofepichlorohydrin and bisphenol A), and possesses essentially the samecoeflicient of thermal expansion as the material used for the tool.Additionally, the insulating material is non-porous, resistant toabsorption of moisture for preventing passage of current through theinsulating material to the workpiece, and relatively chemically inertwith respect to the electrolyte being used. Typical insulating materialsinclude a casting resin type RP-3260 available from Renn Plastics, Inc.,of Lansing, Mich. or STYCAST casting resin type 2651 MM, available fromEmerson and Cuming of Canton, Mass.

In operation, the magnetic chuck 20 is energized and a workpiece 15placed on its forward face 39. The workpiece is displaced byapproximately 0.025 inch from the center of rotation of the chuck alonga line which intersects the center of rotation of the chuck and thefinishing edge 63 of the tool by adjusting the shoes 41 and 42 radiallyinwardly. In the apparatus shown in FIG. 5, the tool is positioned sothat a gap of a first predetermined distance, typically 0.006 inch,exists between its frontal machining surface and the surface of theworkpiece to be machined. In a typical application of this invention,the workpieces are generally preformed to within a predeterminedtolerance so that once the gap distance is established for oneworkpiece, the same gap distance may be used for all workpieces within asingle production run. In the apparatus shown in FIG. 6, the gap isadjusted to approximately 0.001 inch and maintained at this distance bymoving the tool toward the workpiece.

The motor 25 is then energized to rotate the workpiece at a speed ofapproximately r.p.m. and the current density between the workpiece andtool is adjusted to the highest practical level, approximately 6000amperes per square inch to provide the smoothest surface finish and thehighest machining rates. Electrolyte is fed into the gap between thetool and the workpiece at a pressure of ap proximately 350 p.s.i. whichgives an electrolyte flow velocity in the order of 400 to 500 feet persecond. This high velocity flow insures adequate removal of the reactionproducts of the electrochemical machining operation.

With the apparatus shown in FIGS. 2 and 6, the tool may be fed towardthe workpiece at the same rate that the workpiece material is removed tomaintain the gap dimensions constant. In this case, the voltage andcurrent density remain substantially constant throughout the machiningoperation, except during the initial rounding up of the workpiece. Asthe workpiece is machined, its center 50 moves on a line substantaillyparallel to the machining surface of the tool and the center of rotation40. Preferably, the finishing edge 63 of the tool will be closer thanany other part of the tool to the workpiece when it has reached itsfinal dimension. With this apparatus, the tool may also be fixedrelative to the center of rotation, especially when removing smallamounts of workpiece material, for example in the order of 0.003 inch.In this case, the voltage must be increased to compensate for theincrease in the gap dimension if the current density is to be maintainedat a constant high level.

With the apparatus shown in FIGS. 1 and 5, once the maximum currentdensity is obtained the voltage is con- 1 1 tinuously reduced as theworkpiece is machined and moves automatically toward the tool. In thiscase, when the voltage reaches a second predetermined lower value, thisindicates that the gap has been reduced to a second predetermineddistance, and thus the workpiece has been machined to its desireddimensions.

When the workpiece has been machined to the desired dimensions, thecurrent density is lowered for at least one revolution of the workpieceto a second lower predetermined magnitude, typically between 1500 and3000 amperes per square inch, to provide the machined surface with abright appearance, and then the current is abrupty terminated to stopthe electrochemical machining operation. A relay in the power supplycircuit may be used to terminate the current flow abruptly. Since theworkpiece rotates at a relatively high speed, the amount of materialremoved for each revolution is small, in the order of 0.000010 inch, andtherefore when the current is removed, the discontinuity in theworkpiece surface is also small.

FIG. 11 shows the relationship between the voltage and current duringthe machining operation with respect to time. In this example, theapparatus of FIG. 5 was used, however, the same principles of operationapply to the apparatus of FIG. 6. At the start of the electrochemicalmachining operation, the voltage, which is shown by curve 85 in FIG.11a, is increased with a corresponding increase in the current, as shownby curve 86 in FIG. llb. Since the power supply which was used duringthe machining operation of this example had a maximum voltage of 36volts, the current did not obtain the desired high level initially.While this limitation in the capacity of the power supply existed in theembodiment described herein, it is obvious that a higher capacity powersupply could be used to achieve the same results.

As shown generally at 90, the current fluctuates between two valuesindicating that the workpiece is out of round and therefore the gapdistance is constantly changing as a result of the rotation of the part.As the machining operation continues, however, this fluctuationdecreases indicating that the part is becoming round. As describedpreviously, the power supply maintains its voltage constant for at leastone revolution of the part so that the instantaneous value of thecurrent is allowed to vary, thus machining the high spots on theworkpiece at a faster rate than the low spots.

As workpiece material is removed, the gap becomes smaller, and as aresult the current increases slowly until it reaches the firstpredetermined magnitude. The voltage has remained constant during thistime of increase in current. When the peak current reaches itspredetermined high level, shown at 91, it is maintained at that level bycontinuously reducing the power supply voltage as the gap dimensiondecreases. Once the voltage level has been reduced to a predeterminedlevel, shown at 92, indicating that the part has now been machined toits desired dimensions, the voltage is reduced to a second value 93, toreduce the current to a second predetermined level at 94 for at leastone revolution of the workpiece. The voltage is then reduced to zero asquickly as possible, as by opening the circuit by a relay, or byshorting the output of the power supply, to remove the flow of currentand thus prevent any machining at a current density lower than thesecond predetermined current density. This preserves the appearance ofthe surface and its surface finish. Termination of the current in lessthan 100 microseconds is desired in order to minimize the thickness ofthe black line appearing on the surface. A line having a thickness ofless than 0.001 inch is considered acceptable. With presently availableequipment and techniques, the current may be brought to a zero valuewithin approximately ten microseconds.

As mentioned above, when rounding up a part, a power supply having aconstant voltage characteristic for at least one revolution of the partis used so that the curmachined at a lower current density for at leastone rev-- olution to provide a bright" surface finish.

Using the techniques described above, rotating part may be machined towithin 0.0001 inch of a desired diameter, an out of round tolerance inthe order' of 0.000060 inch, and a surface finish of five microinch,arithmetic average.

While the method described herein shows the machin ing of an exteriorbearing surface of a rotating workpiece, it is to be understood thatthis method could also be performed on an inside bearing surface or on afiat surface. The essential steps of this method are therefore themachining at high current densities to provide fast machining ratesand-smooth surface finishes while utilizing a power supply having anessentially ripple freeoutput, and then reducing the current density toa lower value such that the resulting surface finish has a brightappearance.

While the methods herein described, and the forms 0 apparatus forcarrying these methods into effect, constitute preferred embodiments ofthe invention, it is to be understood that the invention is not limitedto these precise methods, and that changes may be made therein withoutdeparting from the scope of the invention which is defined in theappended claims.

What is claimed is:

1. A method for machining a ferrous workpiece toform a surface ofrevolution thereon having a precise final predetermined dimension and asmooth surface finish, said method comprising the steps of supportingthe workpiece for rotation;

placing an electrochemical machining tool adjacent the workpiece to forma gap; rotating the workpiece thereby to move the'surface of theworkpiece relative to said tool;

supplying a high velocity flow of electrolyte into said connecting asource of current having a ripple voltage variation less than one-halfpercent, peak to peak,

between the tool and the workpiece such that the workpiece is anodicwith respect to the tool to cause electrochemical machining of theworkpiece as it moves past said tool; 3'

maintaining the magnitude of the current density at a firstpredetermined level until the workpiece is machined to its finaldimension to produce a smooth surface finish but which has an oxidecoating thereon; momentarily lowering the magnitude of said currentdensity to a second predetermined level for at least one revolution ofthe workpiece to remove said oxide coating thereby to provide a brightsurface finish which is as smoothas the surface finish produced bymachining at'said first current density; and terminating said currentflow in less than microseconds after the entire machined surface of. theworkpiece has been exposed to said second predeter: mined level ofcurrent density. a

2. The method as set forth in claim 1 further including the step ofmaintaining said gap at a predetermined distance throughoutthe machiningoperation by causing relative movement between said tool and the center,of rotation as the workpiece is electrochemically machined.

3. The method as set forth'in claim 1 wherein said gap between said tooland the workpiece decreasesas the 13 workpiece is electrochemicallymachined, wherein said source of current includes a source of variablevoltage, and wherein the step of maintaining the magnitude of saidcurrent flow at said first predetermined level includes the step ofreducing the voltage between said tool and the workpiece as said gapdistance decreases.

4. The method as set forth in claim 1 wherein said gap between said tooland the workpiece increases as the workpiece is electrochemicallymachined, wherein said source of current includes a source of variablevoltage, and wherein the step of maintaining the magnitude of saidcurrent flow at said first predetermined level includes the step ofincreasing the voltage between said tool and the workpiece as said gapdistance increases.

5. The method as set forth in claim 1 wherein the step of adjusting theoutput from said source of current includes the further step ofmaintaining the voltage constant for at least one revolution of the partwhile limiting the peak current density to said first predeterminedlevel to etfect rounding up of the workpiece by remov ing the materialon the workpiece which approaches closer to the tool at a faster ratethan the remainder of the workpiece material.

6. The method as set forth in claim 1 wherein the step of adjusting theoutput from said source of current includes the step of maintaining thecurrent density constant at said first predetermined level to permitremoval of :fixed amounts of workpiece material independent ofvariations in the surface configuration of the workpiece.

7. The method as set forth in claim 1 in which said 14 current densityat said predetermined first level is of the order of 6000 amperes persquare inch, when machining a ferrous workpiece and using a sodiumnitrate electrolyte.

8. The method as set forth in claim 1 wherein the magnitude of saidfirst predetermined level of current density is greater than 3000amperes per square inch and wherein the magnitude of said secondpredetermined level of current density is between 1500 and 3000 amperesper square inch, when machining a ferrous workpiece and using a sodiumnitrate electrolyte.

9. A method as set forth in claim 1 in which said current density atsaid second predetermined level is not less than about 1500 amperes persquare inch, when machining a ferrous workpiece and using a sodiumnitrate electrolyte.

References Cited UNITED STATES PATENTS 3,287,245 11/1966 Williams204-224 3,445,372 5/ 1969 Fromson 2042'l2 3,458,424 7/1969 Bender204-143 M 3,285,843 11/1966 Blake 204-143 M L. C. EDMUNDSON, PrimaryExaminer US. Cl. X.R.

204--129.1, 217, DIG 9

